Why are hydraulic roller lifters, different on different sides in the engines?

Hello folx, new Swedish guy here, & I'm quite confused & sure hope someone can give a seriously good answer...

I just started "stripping for cleaning" 2 -89 318 van engines & they both have different tappets on different sides in the engine!
They're different by the oil-hole on the side & inside there's a ball in one model & a plate in the other but the most confusing I think is that the "big" spring inside at the bottom of the lifters are of different hardness too!!!
& the ball / plate difference makes me wonder if one side gets the oil with higher pressure than the other?

If I wanna swap them for some tune-up brand racing stuff, is all this something worth considering; as in plate / ball + spring-hardness difference?

If they were new, could I put one sort in one engine & ignore the difference?
& what's best: hard-spring ones or softer?
I would imagen harder is better for high-revs but I've been wrong before...

Thanx before-hand & sure hoping for a seriously good & quick answer! ;)
/Nox.